One process utilized in the fabrication of semiconductor integrated circuits is photolithography. Such typically involves reproducing an image from an optical mask in a layer of photoresist that is supported by underlying layers of a semiconductive substrate. In the context of this document, the term "semiconductive substrate" is defined to mean any construction comprising semiconductive material, including, but not limited to, bulk semiconductive materials such as a semiconductive wafer (either alone or in assemblies comprising other materials thereon), and semiconductive material layers (either alone or in assemblies comprising other materials). The term "substrate" refers to any supporting structure, including, but not limited to, the semiconductive substrates described above. The ability to reproduce precise images in a layer of photoresist is crucial to meeting demands for increasing device density.
In a typical photolithographic process, an optical mask is first positioned between a radiation source and a photoresist layer received over underlying layers. The image from the mask is reproduced in the photoresist by exposing the photoresist to radiation from the radiation source through the optical mask. Portions of the mask are opaque and prevent exposure of the underlying photoresist to the radiation. Other portions of the mask are transparent to the radiation, allowing exposure of the underlying photoresist.
The layers underlying the photoresist include one or more individual layers that are to be selectively removed relative to surrounding layers to achieve desired patterning. Depending on the type of photoresist utilized (i.e., positive type or negative type), exposed photoresist is either removed when the substrate is contacted with a developer solution, or the exposed photoresist becomes more resistant to dissolution in the developer solution. In either event, a patterned photoresist layer is formed over underlying layers which can be used as an etching mask in subsequent etching.
One problem experienced with conventional optical photolithography is difficulty in obtaining uniform exposure of photoresist underlying transparent portions of the mask. It is typically desired that the light intensity exposing the photoresist be uniform to obtain optimum results. When sufficiently thick layers of photoresist are used, the photoresist is or becomes partially transparent so that the photoresist where it meets with underlying layers is exposed to a substantially similar extent as the outermost surface of the photoresist. Often, however, light that penetrates the photoresist is reflected back towards the light source from the surface of the underlying layers over which the photoresist is received. The angle at which the light is reflected is dependent on the topography of the surface of the underlying layers and the type of material of the underlying layers. The reflected light intensity can vary in the photoresist throughout its depth or partially through its depth, leading to non-uniform exposure and possible undesirable exposure of adjacent photoresist. Such exposure of photoresist can lead to poorly controlled defined features of the integrated circuit.
In an attempt to minimize the variable reflection of light in a photoresist layer, antireflective coatings have been utilized between the underlying layers and the photoresist layer, or even between the photoresist layer and the radiation source. Antireflective coatings minimize photoresist exposure from surface reflections, allowing the exposure across a photoresist layer to be controlled more easily than the radiation incident on the photoresist from the radiation source.
Antireflective coatings are typically organic materials. Organic layers can, however, lead to particle or other contamination in the integrated circuit due to possible incomplete removal of such organic material from the underlying layers after the photolithography set is performed. Such contamination can, of course, be detrimental to electrical performance of the integrated circuit. Further, the underlying layers upon which the organic materials are formed may be uneven resulting in different thicknesses of the organic material used as the antireflective coating (e.g., thicker regions of the organic material may be present at various locations of the underlying layers). As such when attempting to remove such organic material, if the etch is stopped when the underlying layers are reached, then some organic material may be left. If the etch is allowed to progress to etch the additional thickness in such regions or locations, then the underlying layers may be undesirably etched (e.g., punch-through of an underlying layer can occur).